POLLEN. 



[ 614 ] 



POLLEN. 



ripe, in order to fertilize the ovules. When 

 slightly magnified, the pollen of most 

 flowers appears to consist of granules, of 

 different size and colour in different plants ; 

 hence the individual particles are called 

 pollen-grains or granules (PI. 40). Exami- 

 nation under a sufficient magnifying power 

 shows that the simple or typical forms of 

 pollen-grains are single free cells filled 

 with fluid matter: more complex forms 

 occur in many cases, which, however, may 

 be simply characterized as simple pollen- 

 grains, permanently coherent into definitely- 

 formed groups. 



The pollen-grain may be examined as to 

 its form and stucture, its contents, and its 

 development. 



The forms of simple grains presented in 

 different plants are tolerably varied sphe- 

 rical (PL 40. figs. 8-10, 22, 23, 25) and 

 elliptical (figs. 6, 11, 29) being perhaps 

 those most common ; but besides these, nu- 

 merous geometrical forms occur, such as 

 tetrahedral (fig. 14), polygonal (figs. 16, 27, 

 28), cubic (fig. 19). But it must be noted 

 here that the forms frequently vaiy accord- 

 ing as the pollen is viewed dry or in fluid, 

 since the elliptical and allied forms often 

 expand into a spherical form, when they 

 absorb liquid (figs. 18 & 20 , 6, c). The 

 explanation of this will be given presently. 

 The external appearance is further greatly 

 influenced by minor peculiarities of form, 

 such as ridges, spines and processes of dif- 

 ferent kinds ; these, however, are referable 

 to the structure of the outer coat. 



The ordinary structure of the coats or 

 the cell-wall of the pollen-grain is that of 

 a delicate internal cell-membrane, with an 

 outer, thick and resisting layer, which may 

 be regarded as the CUTICLE of the inner or 

 proper membrane of the cell. In a few 

 cases the inner membrane alone exists, as 

 in the cylindrical pollen-cells of Zostera 

 and some other aquatic plants. In other 

 cases the outer or cuticular coat presents a 

 more complex structure, and two, or, it is 

 said, even three layers may be distinguished 

 in it ; these, however, seem to be merely a 

 lamination of the outer coat. The condi- 

 tions in some of the Coniferae are different 

 from this, and will be alluded to presently. 

 The inner membrane is exceedingly deli- 

 cate and homogeneous : in ordinary spherical 

 or oval grains it accurately lines the outer 

 coat ; in some of those forms which present 

 processes of various kinds, such as (Enothera, 

 it seems to us that the inner coat does not 



extend into these processes in the mature 

 pollen. The outer coat exhibits, as to sur- 

 face, every variety of appearance, from 

 smooth, through granular and spiny, to 

 pseudo - cellular arising from reticulated 

 ridges; in addition to this, the processes 

 just alluded to give a very peculiar aspect 

 to many kinds of pollen. Besides these, we 

 find in all cases markings appearing like 

 pores, or others like slits (which become 

 furrows when dry), or both together, and 

 these in varying number in different cases. 

 The colour of the pollen presents great dif- 

 ferences; although usually yellow, it may 

 be whitish, red (Verbascimi) , blue (Epilo- 

 bium angustifolium) , even black (tulip) : this 

 colour resides in the outer coat. The outer 

 coat also exhibits, in the majority of cases, 

 a secretion upon its surface, of a viscid cha- 

 racter, usually described as oily, but appa- 

 rently consisting of a viscid matter not 

 readily soluble in water, remaining from 

 the dissolved parent cells. It would seem 

 to be the substance which holds together 

 the pollen-grains in those cases where it 

 consists of waxy masses, readily breaking 

 up into small fragments (Ophrydeous Or- 

 chids). In the Onagraceae the pollen-grains 

 are loosely connected by slender viscid fila- 

 ments, which appear to be derived from 

 the same source. 



The more detailed explanation of the 

 character of the pores &c., the projecting 

 processes, and the compound conditions of 

 pollen will be understood better after a 

 sketch of its development. 



The anther, in which the pollen is formed, 

 consists in its younger stages of a minute, 

 solid, cellular- papilla or cylindrical body. 

 At an early period a distinction becomes 

 manifest in its cells : a single vertical row, 

 lying in the position of the axis of each 

 pollen-chamber (or loculus), presents a 

 different aspect, from its cells exceeding the 

 surrounding ones in size ; and these rows 

 undergo a special development to produce 

 the pollen-grains, while the surrounding 

 layers are developed into the tissues forming 

 the coat or wall of the anther, and its mid- 

 rib or connective (see ANTHER). The cells 

 of the primary row multiply by cell-division 

 with the general increase in size of the 

 anther (figs. 592-594), until at length they 

 form relatively large masses of cellular 

 tissue composed of large squarish cells 

 filled with granular contents, well defined 

 as constituting a distinct tissue from the 

 walls of the pollen-chambers. A new 



